Deionized water is used for various purposes, for example, in plants such as for semiconductor production and liquid crystal display production, in industrial facilities such as for pharmaceutical industry, food industry, and electric power industry, even in households, and in laboratories. Electrodeionization apparatuses are frequently used to produce deionized water as described in Japanese Patent No. 1782943, Japanese Patent No. 2751090, and Japanese Patent No. 2699256. A conventional electrodeionization apparatus of FIG. 2 includes electrodes which consist of an anode 11 and a cathode 12, anion-exchange membranes (A membranes) 13 and cation-exchange membranes (C membranes) 14. The membranes are alternately arranged in such a manner as to alternately form concentrating compartments 15 and desalting compartments 16 between the anode and the cathode. The desalting compartments 16 are filled with anion-exchanger and cation-exchanger made of ion exchange resin, ion exchange fibers, or graft exchanger. In the desalting compartments 16, the anion-exchanger and cation-exchanger are in the mixed state or multiple-layered state. In FIG. 2, “17” represents an anolyte compartment, and “18” represents a catholyte compartment.
Ions flowing into the desalting compartments 16 react with the ion exchanger according to the affinity, concentration, and mobility of the ions and move through the ion exchanger in a direction of potential gradient. The ions further pass through the membranes to hold neutralization of charges in all of the compartments. The ions decrease in the desalting compartments 16 and increase in the concentrating compartments 15 because of the semi-permeability of the membranes and the polarities of potential gradient. This means that cations permeate the cation-exchange membranes 14 and anions permeate the anion-exchange membranes 13 so that the cations and anions are concentrated in the concentrating compartments 15. Therefore, deionized water (pure water) as product water is recovered from the desalting compartments 16.
Electrode water flows through the anolyte compartment 17 and the catholyte compartment 18. The water flowing out of the concentrating compartments 15 (concentrated water) and having high ion concentration is as the electrode water in order to ensure the electric conductivity.
Raw water is introduced into the desalting compartments 16 and the concentrating compartments 15. Deionized water (pure water) is taken out from the desalting compartments 16. Concentrated water in which ions are concentrated is discharged from the concentrating compartments 15. A part of the concentrated water is circulated into the inlets of the concentrating compartments 15 by a pump (not shown) in order to improve the product water recovery. Another part of the concentrated water is supplied to the inlet of the anolyte compartment 17. The reminder of the concentrated water is discharged as waste water out of a circulatory system in order to prevent the ion concentration in the circulatory system. Water flowing out of the anolyte compartment 17 is supplied to the inlet of the catholyte compartment 18. Water flowing out of the catholyte compartment 18 is discharged as waste water out of the circulating system.
The pH in the anolyte compartment 17 is lowered due to H+ generated by dissociation of water. On the other hand, the pH in the catholyte compartment 18 is increased due to generation of OH−. Thus, the acid water flowing out of the anolyte compartment 17 is introduced into the catholyte compartment 18 so that alkalinity in the catholyte compartment 18 can be neutralized, thereby eliminating damages due to scale formation.
There have been various reports indicating that the quality of the product water can be affected by the concentrated water in the above conventional electrodeionization apparatus. Filling activated carbon or ion-exchange resin into electrode compartments is disclosed in U.S. Pat. No. 5,868,915.
The above conventional electrodeionization apparatus do not remove silica and boron at extremely high ratio. For example, it is very difficult to remove silica at a rate of removal of 99.9 to 99.99% or higher.
There have been reports indicating that the concentrated water affects the product water, but there have been no reference to the relationship to silica and boron. Filling activated carbon or ion-exchange resin into electrode compartments can reduce the electrical resistance, but it can not reduce silica and boron.